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s. s. KISTLER May 25, 1954 2,679,545 HIGH-TEMPERAT RE RESISTOR AND METHOD OF OPERATING IT 2 Sheets-Sheet 1 Filed Sept. 15, 1950 I, ll 42 Jwveamar 579M051. 5. K/JTLER May 25, 1.954 s, 5, K|5TLER 2,679,545

HIGH-TEMPERATURE RESISTOR AND METHOD OF OPERATING IT Filed Sept. 15, 1950 2 Sheds-Sheet 2 TEMPERATURE (DEG. K) IOOOIEOO I400 I600 I600 2000 50 l l l I 50 40 40 SOURCE OF AI.

30 AMBIENT TEME 30 I |o00K CENTIMETER CURRENT 20 20 REGu LAToR I0 :0 HM V AMBIENT TEMP O l 0 I600 I600 2000 2200 2400 2600 TEMPERATURE (DEGK) Hg. 4

CRTCAL 2 TEM PERATURE v 8 l b I 5511 800 1000 I200 I400 I600 I800 AMBIENT TEMPERATURE(DEG- K) H EAT I 4 (WATTS) IWI/ f UfOT 20 SAMUEL 5,v K15 TLER AMBIENT TEMPERATURE (DEG. K) i Patented May 25, 1954 g 9 2,679,545

UNITED STATES PATENT OFFICE HIGH-TEMPERATURE RESISTOR AND METHOD OF OPERATING IT Samuel S. Kistler, West Boylston, Mass., assignor to Norton Company, Worcester, Mass., a corporation of Massachusetts Application September 15, 1950, Serial No. 184,926 14 Claims. (01. 1320) The invention relates to high temperature elecmany oxides are electrical conductors at elevated trical heating resistors. temperatures, these oxides in general have nega- One object of the invention is to provide an tive temperature coeificients of resistance, eselectric furnace capable of reaching high tempecially those which seem from all standpoints peratures and of operating at hightemperatures to indicate usefulness as heating resistors. Since over a long period of time. Another object of zirconium oxide, also referred to as zirconia the invention is to provide an electric furnace doesnt melt until the extremely high temperawith an oxide resistor which will not channel. Anture of about 2700 C. is reached, it would at first other object of the invention is to provide a reblush appear to be an excellent material for a sistance element of such material, shape and heating resistor; but besides having a negative characteristics that it can be operated at high temperature coeificient of resistance, it undergoes temperatures without channeling and without a sharp volume change upon heating and cooling. burning out. Another object of the invention is Also it is such a poor conductor at low temto provide cold end portions for an oxide heating peratures that it must be heated by another element. 15 source of heat before it will operate. Hence when Another object of the invention is to produce used as a heating resistor, the furnace has to be practical embodiments of oxide heating resistors. heated by another means in order to make the Another object of the invention is to provide zirconia resistor or resistors conducting. Furheating resistors and furnaces utilizing them of thermore the negative temperature coeflicient reparticular utility in research laboratories for sults in a decreasing resistance as the temperathe attainment of high temperatures. Another ture rises and on a constant potential source the object is to provide high temperature resistors resistor either becomes progressively hotter and that can be used in many different atmospheres. hotter until it burns out or it cools off and Another object is to provide a high temperature ceases to conduct. Moreover the negative temelectric furnace for operation at temperatures 5 perature coefficient of zirconia has caused rods above 1600 C. and even above 2000 C. made thereof to become molten along the axis Other objects will be in part obvious or in part because the current always tends to flow Wherpointed out hereinafter. The invention accordever the material is hottest. This phenomenon ingly consists in the features of construction, is known as channeling and has been pronounced combinations of elements and arrangements of in cases where furnaces have been provided with parts, as will be exemplified in the structure to large oxide heating resistors. Channeling can be hereinafter described and the scope of the also occur in tubular resistors, the material heatapplication of which will be indicated in the foling more along one side or axial strip, with conlowing claims. sequent failure. Other diificulties have hereto- In the accompanying drawings, illustrating fore been encountered. One of the objects of two of many possible embodiments of the methis invention is to overcome such disadvantages chanical features of this invention: and difiiculties.

Figure 1 is a vertical sectional view of a fur- Referring now to Figure 1, a furnace constructnace having a resistor constructed in accorded in accordance with the present invention may ance with the invention, comprise a furnace body [0 made up of any suit- Figure 2 is a vertical sectional view of an able refractory material for example out of kaolin other furnace having a resistor constructed in bricks. This furnace body It! provides a chamber accordance with this invention to illustrate an- H in which the heat is developed by a zirconia other embodiment, resistor heating element I2 which as will be ob- Figure 3 is a plan view of the furnace of Figure served is in the form of a tube.

2, The zirconia tube I2 is made out of stabilized Figure 4 is a graph illustrating stable and unzirconia ZIOz preferably stabilized with from stable conditions in such a furnace under dif- 3% to 6% lime, CaO, and the material may be ferent loads and at different temperatures, made in the following manner. An electric arc Figure 5 is a graph of critical temperatures for furnace of the type disclosed in U. S. Letters a zirconia resistor in different ambient tempera- Patent No. 775,654 patented November 22, 1904 to tures, Aldus C. Higgins is provided. Furnaces of this Figure 6 is a graph of the heat radiated per type comprising iron shells cooled all over with a square centimeter of a zirconia resistor at the cascade of water have been in use practically ever critical temperature for different ambient temsince the date of the above patent and are well peratures, known to electro-chemists and therefore need not Figure 7 is an electrical diagram. be further described herein. A furnace mix- As conducive to a clearer understanding of certure of zirconia ore, coke, iron, carbon, and tain features of the present invention it is pointlime (CaO) is prepared. Various zirconia ores or ed out that while it has long been known that partially purified zirconia powder can be used.

In general these are the zircon and baddeleyite ores. However chemically purified zirconia can equally well be used but is of course more expensive.

The quantity of carbon provided in the furnace mixture should be two-thirds of the theoretical quantity of carbon required completel to reduce the silica plus of the theoretical quantity required to reduce all the other'oxides (except the zirconia) to metal plus about excess over all of these quantities. This quantity can be varied from the above with no excess to the above or with excess. The reason why only two-thirds of the theoretical quantity of carbon required completely to reduce the silica is provided is that about one-third of the silica is volatilized during the furnacing operation. On the other hand the excessmentioned is provided because some of the coke is used up by combining with oxygen other than that provided by the oxides to be reduced.

Th quantity of iron should be enough to form with the silicon that is reduced from silica a ferro-silicon having an iron content of from 75% to 85%. The purpose of the iron is to combine with the silicon to form a ferro-silicon alloy which has a much higher specific gravity than elementary silicon and therefore will go to the bottom of the furnace and, after solidification, form a ferro-silicon button containing also other reduction products that can readily be separated from the rest of the ingot. The amount of iron to add is enough to make with two-thirds of the silicon present in the ore a ferro-silicon having an iron content of from 75% to 85% minus the amount of iron obtained by the reduction of the iron oxide in the ore to iron and this of course must take into account that a small percentage of iron oxide remains in the final product.

The quantity of lime as a stabilizing agent to be added should be from 3% to 6% of the amount of ZI'Oz in the ore. Th reason for providing the stabilizing agent in the above percentages is that less will not satisfactorily stabilize the zirconia, 'and more will form a eutectic thus making the product less refractory. The stabilizing agent in the range given causes the zir-. conia to crystallize predominantly in the cubic system, but when less of the stabilizing agent is used the crystals are predominantly monoclinic. Ordinary or natural baddeleyite is monoclinic whereas the product of this invention is predominantly cubic. The monoclinic form of zirconia will not withstand many cycles of heating to over 2000 C. and cooling to 100 C. and even lesser temperature changes may cause cracking or fracturing of the heating element if it is made of monoclinic zirconia. A zirconia of predominantly cubic crystal form will, however, withstand heat shock for many cycles. When the lime is as much as 6% the crystals are nearly all cubic, when the lime is as low as 2.7% about 35% of the crystals are cubic. Zirconia having 3% to 6% of lime on the Z1O2 is referred to as stabilized zirconia; the expression stabilized means that the zirconia does not have a detrimental volume change at the critical temperature of inversion of baddeleyite, normallyabout 1000 C. However I do not want to be limited to lime stabilized zirconia nor to the precise furnace operation described herein since magnesium oxide (3% to 6%) is likewise a stabilizing agent and other processes of combining the stabilizing agent with the zirconia can be effectively used. However the best material now known to me-for tube 6 inches long with a 1 /2 the manufacture of the resistor heating elements I2 is that above described.

For the manufacture of tubes of zirconia I may take zirconia grain and slip cast it, ram it, or extrude it to form the green shapes, then dry the shapes and fire them at cone 35. However a very convenient way is to press the material to produce the tube shapes. The grit size of the zirconia has som relation to the size of the tube, for example it is not desirable to try to make very-small tubes with coarse grit. However this matter is not critical and the ceramist will readily know how to select proper grit sizes especially in view of the following example:

For the manufacture of a stabilized zirconia inch diameter bore and a 1%. inch outside diameter, I selected stabilized zirconia of grit sizes 24 mesh and finer down to 2 microns. This Was simply the material produced by grinding the zirconia until it would pass a 24 mesh screen. This grain was wet with a solution of 2% water and 1% dextrine, the percentages being on the weight of the zirconia. This wet mixture was then charged into a rubber mold having a steel arbor and was pressed at 5000 pounds to the square inch. The molded green tube was then stripped from the mold, was dried and was fired at cone 35. This tube containing about 5% of lime on the total ZrOz made a highly satisfactory heating resistor.

Referring again to Figure l, the resistor I2 is seated in intermediate cold ends l3 which are likewise made of stabilized zirconia and can be fashioned out of the material above described in the manner above described but steel molds can be used as it is not necessary to use a rubber mold to mak cylindrical blocks such as the cold ends it. These cold ends l3 have approximately four times the cross section of the tube l2 and consequently should have one-fourth the resistivity of the tube 32 at the same temperature. Within the scope of my invention the cold ends it should have at least twice the cross section of the tube l2 and I cannot give an upper limit because the greater the cross section of the cold ends it the better will be the performance of the furnace, only there are of course practical limitations on indefinitely increasing the size of these cold ends.

In contact with the intermediate cold ends l3 are outer cold ends M made of material which at practically any temperature has a lower rc sistivity than that of zirconium oxide, stabilized or otherwise. These outer cold ends l4 may, as shown, b cylindrical blocks and preferably they have nicely ground plane surfaces [5 in contact with equally nicely ground plane surfaces It on the ends of the cold ends l3. These outer cold ends it may be made of any oxide selected from the group consisting of zinc oxide ZnO, titanium oxide TizOs, magnetic iron oxide F6304, copper oxide CuzO, manganese oxide M11304, and uranium oxide UOz. However from all standpoints including especially cost, I prefer zinc oxide, ZnO.

It is preferable that the oxide used be not very pure. The listed oxides are all of them more conducting if some impurities are present since defects in the crystal lattice, promoted by strain from foreign atoms or a deficiency or excess of one of the atoms of the oxide, considerably increase the conductivity. Mixtures of any two or more of these oxides mentioned can be used and reduced products are certainly not excluded, but on the other hand are desirable and foreign matter of almost any kind is not detrimental and naturally any free metals present increase the conductivity. It is therefore not possible to give limits on the purity of the material of the outer cold ends [4 since clearly if copper oxide contained so much free copper present that there was only 13% of CuzO, it would be more conductive than pure Cu2O. But on the other hand the purpose of using an oxide at all for the outer cold ends would be defeated by using almost pure metal so therefore the only definition I can give is the melting point of the material. The melting point of the outer cold ends should be no lower than 1200 C. The following table giving the melting points of metals and oxides may be helpful in this connection.

TABLE Melting points of metals and oxides Manganese. l Titanium Uranium d decomposes.

As a specific example of the composition for the outer cold ends I4 I took a quantity of granular zinc oxide ZnO, technical grade, mixed it and fired them to 1300 C.

Referring again to Figure 1, the outer cold ends I4 should be resiliently pressed against the intermediate cold ends l3 and this can be done in any manner. As shown the furnace body In rests upon a steel plate I! and to this steel plate H are fastened L-shaped springs l8 by means of screws l9 extending through slots in the ends of insulating plates 2|, the screws [9 being beyond the sides of the horizontal portions of the springs l8 so that the springs l8 are insulated from the plate H. The upper ends of the springs [8 contact the outer ends of leads 22 that extend into the cold ends l4 as shown and have a snug fit therein. These leads 22 are preferably made of a nickel iron chromium alloy such as with .1 to .7 carbon.

It will be seen that the outer cold ends I4 are entirely outside of the furnace body l0 although they Consequently desired shape but for convenience in constructing it as well as in using it, it may have a rectangular parallelepipedal shape. At least on one side of the chamber II and preferably on each side thereof is a removable brick 25 for gaining access to the chamber II and for admitting a flame thereto and exhaust gases therefrom in order to start the furnace in operation.

Electric current can be conveyed to the leads 22 via the springs [8 by means of copper conductors 21 connected to the springs I8 by binding posts 28 having nuts 29.

Figure 2 illustrates another embodiment of the invention in which the zirconia resistor heating element 32 also functions as a furnace wall. In this embodiment there are intermediate cold ends 33 which have an annular shape and these are in contact with outer cold ends 34 also having an annular shape. The outer cold ends 34 have nicely ground plane surfaces 35 in contact with equally nicely ground plane surfaces 36 on the intermediate cold ends 33. The lower cold end terial. Just above the batt 31 is an annular plate 39 which may be made of sintered alumina. Inserted into and extending radially from the outer cold ends between the batt 3'! and the annular plate 39 are a plurality of leads 42 which may be made of the same nickel iron chromium alloy as above described and similar leads 42 are imbedded in and extend radially from the upper outer cold end 34.

Just outside of the heating element 32 and resting upon the lower intermediate cold end 33 is a heat retaining zirconia tube 43, preferably made of the same stabilized zirconium oxide as drical steel shell 44. Between the steel shell 44 and the zirconia tube 43 is packed a quantity of zirconia grain 45 which may be of the stabilized variety or otherwise. Resting on this grain 45 and surrounding the upper intermediate cold end 33 is a short tube 46 made of sintered alumina.

same stabilized zirconia. This cap 48 has a sight hole 49. g

The annular plate of alumina 39 rests upon a steel ring 50 having radial holes through which extend the leads 42. There is another ring 50 at the top of the furnace which is clearly illustrated in the plan view of Figure 3. Strip metal springs 52 are secured by screws 53 to the top ring 50 and also to the bottom ring 50 and the ends of the springs 52 engage the outer ends of the leads 42 thus causing the latter to exert pressure against the cold ends 34 in which they are imbedded, for the purpose of making good electrical contacts. Electric current can be conveyed to the rings 50 and thence to the leads 42 by means of conductors 55 attached to the rings 50 by screws 56. In Figure 3 a cable 57 is shown connected to the conductor 55 and, of course, there similar conductor attached to the lower ring 50. Illustratively, the dimensions of the tube 32 of Figure 2 may be a length of 6 inches, an inside the resistor tube is more nearly and in the construction shown) diameterrof 3%; inches, andan outside diameter of 1% inches. It may be constructed in the 'manner. above described in connectionwith the makingof tube if. of Figure l, illustrative dimen sions-for which Ihave also given above. These dimensional relations between diameter and wall thicknesses are predetermined according to my invention .50 that the wall thickness of thezir- :coniatube is not too thick relative to the'diameter ofthe tube and meets the criterion that ID f. t v Wa is no less than 2 where I!) is theinside diameter of the :tubeand -Wais the wall thickness. 'of-my invention -practical advantagesand results heretofore impossible so far as I am aware.

As above indicatedthe end faces or surfaces of the tubular element it of Figure l and of the tubular element 32 of Figure 2 are preferably shaped or faced, as by grinding, to give even and uniformsurface contact with the intermediate blocks or cold ends i3,-l3 in Figure l and 33, 33 -in Figure 2, and the latter may also be similarly faced or treated on those portions where they engage the resistor tube, thus to aid in achieving uniform contact resistance throughout the engaged surfaces .wherebysubstantial uniformity of current flow per unit area of the cross-section of achieved. Each resistor tube, being a hollow cylinder, may be regarded as comprising a number of individual longitudinal resistor elements arranged in a circle and in intimate side wall contact with each other and each carrying the same amount of heating current therethrough, and if the resistor tubes are dimensionally within the above criterion, it is possible to achieve'operation of the resistor and of the furnace without materially disturbing the just described uniformity of cur rent flow through subdivisions or these imaginary longitudinal elements of the hollow cylinder,

.as .will be later described.

Uniformity of the surface contacts just men- :tioned above and also of the surface contacts between theintermediate cold ends and the outer .cold ends whose contacting faces are also preferably ground, as above noted, further enhanced by the continuity of the yielding pressures with which these serially successive surfaces in each of the illustrative furnace constructions are held in engagement with each other. In the construction of Figure 1 the spring arms l8, l8 supply and maintain this contact pressure, of Figure 2 it is the weight of the successively stacked parts that furnishes this continuous but yielding pressure. Moreover the feature of yieldability also permits the various pressed-together parts to partake of dimensional changes in response to temperature changes, all without subjecting any of the parts to detrimental stress and without disturbing uniformity and continuity of application of the contact pressure.

The furnace may be energized from any suitable source of current, preferably an alternating current source, as is diagrammatically indicated in Figure 'l, and to the output thereof the furnace of Figure l or the furnace of Figure 2 may be connected through a suitable switch (not The electrical system comprises any constancy of current poses of illustration I have indicated in Figure '7 .ing employment in perature T above suitable -means for automatically maintaining supply thereto and for purany suitable form of constant current regulator, preferably one in which the standard or magnitude of the current value to be kept constant may be manually set.

As previously stated, the furnace of Figure 1 can be started by a. gas flame introduced through the hole which is left when a brick 25 is removed, the exhaust gases emerging through the hole on the other side the other brick 25 being also removed. I have found that it is sufficient to heat the element I? to about 1400" C. to start the furnace as after that the element l2 willcarry enough current to continue the heating and raise the temperature gradually to that desired. The furnace of Figure 2 can conveniently be started by removing the hearth block 41 and the cap 48, placing a silicon carbide resistor vertically through the entire furnace (the batt 31 having a hole 59 in the bottom, as shown), energizing the silicon carbide resistor, thus heating the element 32 to about 1400 C., then removing the silicon carbide heating element, replacing the hearth block 4'? and the cap A8, and turning on the current through the element 32.

According to my invention, I am enabled to achieve such substantial freedom from hazards and deficiencies heretofore met with as to make possible the reliable operation of oxide heating elements of large surface area. In operating the furnace, according to my invention, the initial heating of the tube element is effected'in a manner to avoid giving rise to ruinous conditions of instability and, once it is heated and the electric current applied, I again effect controls and conditions that avoid instability, and in these connections the dimensional criterion of the tubular heater element plays a part.

If a constant potential is applied to the resistor and the furnace is heated. by some means, such as a gas burner, until the resistor becomes conducting, the conditions are ingeneral unstable and,

. without more, the temperature will increase until the element is destroyed. On the other hand, if the applied voltage is so controlled that the temperature does not run away, thelikelihocd is that one side of the tube will become Warmer than other parts, thus becoming a better conductor leading to a heavier current therethrough which in turn leads to still higher temperature until virtually all of the current is channeling down one side of the tube which usually results in breakage. Itis this sort of thing that has been one of a number of major factors heretofore limiting the uses of conductive oxides and prevent- .heating elements of large surface area,

According to my invention, these obstacles are overcome. I have investigated the factors involved therein and as of aid in describing how I overcome the above obstacles, let it first be assumed that an element, sufficiently small in diameter to heat uniformly throughout the cross section (say, on the order of 2 millimeters), is enclosed in a furnace maintained at a constant temperature To. If a small current is passed through the element, heating it to some temthat of the furnace, it will lose heat to the furnace through convection and radiation. At least, as a first approximation, one canreasonably represent the heat loss by the equation tains such factors in which H represents the heat lost per square centimeter in watts, 7c is the heat transfer coeflicient through convection, and 7c is the heat plotted in Figure 4 for two furnace temperatures, 1000 K. (abscissae at the top) and 1600 K. (abscissae at the bottom). It is assumed,

cross section.

The specific resistance of lime-stabilized zirconia has been measured over a ture range, and while there are variations due to impurities, porosity, and grain size distribution, it can be represented reasonably well by the equation logw R 2.82

Both curves of Figure 4 gothrough a definite maximum, although the one for Tu=-1000 K. rises much more steeply and attains its maxithan the other. At any teming in more energy ated, and the temperature will fall back to the equilibrium position. On the other conditions, since any accidental displacement of the temperature upward will cause a greater generation of heat than can be radiated and the temperature will rise at a progressively faster rate until fusion occurs.

Furthermore, the element is permanently un- To is increased, so that a potential that is safe in which, for stabilized zirconia, A=0.00152, and 3:13.700. Note that A, which conas porosity, grain size, shape according to my invention, initial zirconia tube is efiected as above described, as

r tubular element at uniform of cross section of the element, and magnitude of the specific resistance, does not appear in Equation 3.

Figure 5 shows that at low temperatures the critical temperature is very little above the ambient temperature, thus providing only a narrow region of stability, while at high furnace temperatures the stability limits are very wide. At furnace temperatures above 1800 K., instability is no longer possible with a voltage below that necessary to melt the zirconia element.

Thus critical temperature and. safe radiation desiderata are determinable for the oxide resistor whose wall thickness should not be great enough to permit detrimental or excessive radial temperature gradients, and specific criteria are above illustratively set forth for zirconia. N 0 such radial tubular heating element, of small-cross-sectioned parallel elements equally and uniformly energized electrically, and the conditions for stability within that assembly so energized are the same as those described above. Moreover, the thin-walled tube used in these calculations also meets the criterion for wall thickis in effect an assembly furnace. It has been amply confirmed in experimental furnaces.

Accordingly, in operating furnaces constructed heating of the by using gas or a temporarily inserted resistance element, the zirconia tube being heated substantially uniformly to a temperature on the order of 1400 C., whereupon the tubular heating element is connected to the power supply and control circuit as above described. The .zirconia heater tube meets the criterion of wall thickness above set forth, and the provisions above described insure that the current flows into and out of the current density throughout the above described contact faces or surfaces at the respective ends thereof, and this action is assured though the parts partake of temperature responsive dimensional changes. The preliminary heating, as by gas or an inserted resistor, has made the heater tube sufiiciently conductive for the flow of current from one end to the other and the current flows therethrough at substantially uniform density throughout any cross section taken between the ends of the heater tube. It functions in effect like a set of smallcross-sectioned individual elements arranged integrally in a circle, and because the above relationship of wall thickness to inside diameter is not departed from, if, any one of these imaginary elements or subdivisions of the zirconia tube heats up beyond the others, it can and does lose more heat by radiation than it gains, and thus the conditions that are otherwise conducive to channeling are counteracted. Thus continued heating up of the heater tube is facilitated, as is also subsequent operation of the furnace at the desired ultimate furnace temperature.

This action of continued heating up after the current is turned on will thus also be seen to contribute toward maintaining uniform current density, and thus uniform heating up is reliably and more quickly attained. The action just above described might be said to achieve substantially automatic maintenance of temperature equilibrium throughout themass of the heater, and a similar substantially automatic action takes place during the run of the furnace at its ultimate furnace temperature at which treatment of'articles or materials is desired to take place.

It is preferred to supply energy to the hollow cylindrical element of the furnace under conditions of constant current control, effected by setting the current regulator of the supply system of Fig. 7 to the desired current value, the regulator being manually set to function at the selected standard or value of current to be kept constant. It is possible, once the heater has been sufficiently heated by an extraneous source, to close the circuit and set the regulator at a current value that willultimately give the desired-operating furnacetemperature upon com-- pletion of the heating up by the current itself, for the above-described dimensional criterion or characteristic of the zirconia tubular heater and the limitation of current or amperage effected by the current regulator coact to achieve prevention of conditions of instability such as those described above in connection with Figure 4. Be-

cause of such coactions and by setting the current regulator to successively different standards of operation, it is also possible to supply current to theheater tube at a different value during the heating-up period than is supplied during the continued operation of the furnace at its desired temperature, so long as the change in value-is effected with due'regard to the otherwise inherent conditions of stability and of instability that exist, respectively, to the left and to the right of the maximum points of the graphs, such as those of Figure 4, for the heater element. Because of the wide range of stability that exists to the left of the knee in the graphs, preciseness and closeness of control can be departed from and energization; throughout that range of stability, could even be effected at higher current values or at varyingcurrentvalues such as accompany the application of the electrical energy at constant voltage, and in this latter connection the energy could be applied in successive stages at successively different standards of constant voltage, if desiredprovided, of course, as will now be clear, that substantial precision and closeness of constancy of current be'instituted just before or when the critical temperature is reached, for thereafter, as above explained and demonstrated, conditions of instability can be brought about at potentials and temperatures to the right of the maximum point in the corresponding graph like that of Figure 4.

plai-ned and illustrated above However, once the current is turned on after the preliminary heating by an extraneous source, it generally will be preferable to maintain constancy of current regulation at a single and appropriate current value which, for'the particular tubular heater element, will insure continuity of conditions of stability throughout. In this preferred method, the further advantage is achieved in that the continued heating up that goes'on after the removal of the extraneous heat source takes place relatively slowly, which is desirable because it lessens any tendency toward non-uniform heating up throughout the cross section of the tubular element; the selected current value, of course, is one that is within the limits of the conditions for stability to the left of the maximum point or points in the graphs of Figure 4, and While, as earlier explained above, a voltage that is safe at a low furnace temperature may become. excessive at higher temperatures, the voltage at which the selected current value is delivered to the heater unit after the ultimate furnace temperature is reached is, therefore, well within a safe value during the heating-up stage. For example, for the furnace of Figure 2, an ultimate furnace temperature of 1600 C. may be desired and in that case, after preliminary heating to about 1400 C., the current is turned on with the regulator set to maintain current flow constant at 20 amperes. After turning on the current, so that the heating up proceeds at a constant current flow of 20 amperes, the voltage drops slowly to a value of about 127 volts and the furnace temperature levels off at 1600 0., giving a power input of about 2540 watts. To illustrate the functioning of the intermediate and end blocks, the temperature of the intermediate block was about 1030 C. (measured at the bottom. of a hole therein one inch deep), and the zinc oxide end block had a temperature of 985 C.

Thus it will be seen that, by my invention, many thoroughly practical advantages are successfully achieved. I have operated furnaces constructed in accordance with this invention. at temperatures between 1600" C. and 1800 (3., over substantial periods of time and without any trouble or difiiculty. The risks, damage, losses, and limitations imposed or caused by channeling can be successfully avoided and controlled stability achieved according to the principles of my invention. While, so far as concerns the resistor element or'elements per se', I have described my invention in connection with the use of zirconia resistor elements, and more particularly stabilized zirconia heater elements, I have done so because of the over-all superiority and many novel advantages that I am enabled thereby to achieve, but I do not wish to be lirnted thereto except as such limitation may be expressed in one or more of the claims. Many of the advantages and various of the results of my invention may be achieved by utilizing other known refractory oxides that are conducting at high enough temperatures. Of course, elements of expense or cost of some of these will enter as a consideration in their practical application. Also, the particular characteristics of any one or more of such other refractory oxides might make it more suitable for some particular purpose; for example, for very high temperatures, thoria may be employed ir preference to zirconia'because, for example, it ha: a higher melting point. Whatever the selectec refractory oxide, stability of operation may b achieved according to the principles fully ex in connection witi the use of stabilized zirconia, as for example, by shaping it in the form of a tube in which the relation between the inside diameter and wall thickness is such that detrimental or excessive radial temperature gradients are not produced, and supplying electrical energy thereto so controlled that ambient temperature and resistor temperature are maintained within those ratios that mean stability, as is illustratively explained above in connection with stabilized zirconia tubular resistors.

It will also be seen that some known disadvantages or limitations met with heretofore are described. Nor do I Wish making of the outer cold ends, to the oxides I have above mentioned specifically, except as any such limitation is set forth in any of the claims; other known conductive refractory oxides are also usable in the making of action with other parts.

It will thus be seen that there has been provided by this invention a high temperature resistor in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the invention and as many changes might be made in the embodiment above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. A high temperature electric furnace comprising means forming a refractory heat-resistant enclosure having therein a tubular heater element made of a refractory oxide that hasa negative temperature coeflicient of resistance, said tubular element being substantially non-channeling, having a substantially uniform cross section throughout its length and having a wall thickness less than that at which, throughout the throughout the 360 prising a succession in massive per unit length compared to the mass of said tubular element per unit length and each quantity on the order of from about 3% to about 6 of the amount of zirconia. 3. An electric furnace as claimed in claim 1 alkaline oxide selected from the group consisting of lime and magnesia.

4. An electric furnace as claimed in claim 1 in which the inner block of each of said succescurrent-distributing said 360 extent of the contacting surfaces.

6. A high temperature electric furnace comprising a refractory heat-resistant enclosure and neling in a high temperature electric furnace resistor element that is of a refractory losttby the element throughsits surface, heating the oxide resistor element substantially uniformly by means of an extraneous source of heat until it becomes significantly conductive, then supplying electrical energy thereto uniformly distributed to it at its respective ends and throughout the 360 extent of the latter and at a relatively low rate to slowly heat it up to operating temperature and in so doing maintaining the applied voltage of the current within the voltage factors of stability of the oxide resistor element for the respective successive heating-up temperatures, and thereafter supplying the oxide resistorelement with current at relatively high amperage andat'relatively low voltage within the voltage factorszof stability of the oxide resistor element for the temperature of 1 continued furnace operation.

8. Ahigh temperature resistor unit for electric furnaces comprisinga heater element in the form of a hollow cylinder made of a refractory oxide that has a negative temperature coefficient of resistance with means for making electrically conductive contact with at least one end of said cylinder, said means comprising a member of materially greater cross section than that of said hollow cylinder and made of a refractory oxide of substantially the same negative temperature coefficient of resistance as that of said hollow cylinder and having a face for surface contact with an annular face of said hollow cylinder whereby pressure of surface contact therebetween places said refractory cylinder under stress in axial direction andsaid refractory cylinder is free of stressing in radial direction, and a memher in surface engagement with said firstmentioned member and made of a material which has a lower resistivity than that of said first member andthat is selected from the group con sisting of zinc oxide, titanium oxide, magnetic iron oxide, copper oxide, manganese oxide, and uranium oxide.

9. A resistor unit comprising a tubular element made of a refractory oxide that has a negative temperature coefficient of resistance, said tubular element being of substantially uniform cross section throughout its effective length and having a wall thickness not greater than that at which, throughout the desired operating temperatures and in response to electric current flow therein, F

heat is generated internally of the refractory oxide forming the wall of said tubular element at a rate greater than the rate at which said tubular element loses heat from its wall surface, thereby to resist channeling, and a element abutting against the annular end face of said refractory oxide tubular element and made of a similar refractory oxide and of materially greater cross section than that of said tubular element and having a seat providing a surface of 360 extent for making surface contact with a surface of 360 extent at an end of said hollow cylinder for substantially uniformly distributing current to said end.

10. A resistor unit as claimed in claim 9 in which said cold end element comprises a solid cylinder of said refractory oxide and said seat comprises an annular recess in an end of said solid cylinder to receive an end portion of said hollow cylinder.

11. A resistor unit as claimed in claim 9 in which said "cold end element comprises a ringshaped member of substantial radial dimension compared .to the thickness of the wall of said hollow cylinder and said seat comprises a sub cold end 16: stantiallycoaxial annular recess for receiving an end portion of said hollow cylinder.

12. A high temperature resistor for arr-electric furnace comprising a tubular element of refractory oxide selected from the group consisting of zirconia and thoria, stabilized, containing from 3% to6% of alkaline oxide selected from the gl'Oilpconsisting of lime and magnesia, said tubular element being of substantially uniform cross section throughout its effective length and having a wall thickness not greater than that at which, throughout the desired operating temperatures and in response to current flow therein, heat is generated internally of the wall mass of said stabilized refractory oxide at a rate greater than the rate at which said tubular element loses heat from' its wall surface, thereby to resist channeling.

13. A high temperature resistor for an electric furnace comprising a tubular element of stabilized zirconia comprising the reaction product under heat of sirconia with lime of a quantity on the order of from about 3% to about 6% of the amount of zirconia, said tubular element being of substantially uniform cross section throughout its effective length and having a wall thickness not greater than that at which, throughout the desired operating temperatures and in response to current flow therein, heat is generated internally of the wall mass of stabilized zirconia at a rate greater than the rate at which said tubular element loses heat from its wall surface, thereby to resist channeling.

14. A high temperature resistor for an electric furnace comprising a tubular element of stabilized Zirconia comprising the reaction product under heat of zirconia with an oxide selected from the group consisting of magnesia and lime of a quantity on the order of from about 3% to about 6% of the amount of zirconia, said tubular element being of substantially uniform cross section throughout its effective length and having a wall thickness not greater than that at which, throughout the desired operating temperatures and in response to current flow therein heat is generated internally of the wall mass of stabilized zirconia at a rate greater than the rate at which said tubular element loses heat from its wall surface, thereby to resist channeling.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 634,296 Nernst et al Oct. 8, 1901 775,664 Higgins Nov. 22, 1904 847,003 Von Ischewsky Mar. 12, 1907 1,132,684 Queneau Mar, 23, 1915 1,218,058 Cope Mar. 6, 1917 1,306,878 Thomson July 8, 1919 1,352,387 Saunders Sept. 7, 1920 1,438,936 Elmer Dec. 12, 1922 1,470,195 De Roiboul Oct. 9, 1923 1,576,621 Anderson Mar. 16, 1926 1,613,877 Dyckerhoff Jan. 11, 1927 1,969,132 Heyroth Aug. 7, 1934 2,231,723 Jung et al. Feb. 11, 1941 2,268,691 Brooke Jan. 6, 1942 2,294,034 Jaeger Aug. 25, 1942 2,320,172 Brooke et a1. May 25, 1943 2,323,051 Junker et al. June 29, 1943 2,404,060 Hall et a1. July 16, 1946 2,516,570 Hartwig et a1 July 25, 1950 

12. A HIGH TEMPERATURE RESISTOR FOR AN ELECTRIC FURNACE COMPRISING A TUBULAR ELEMENT OF REFRACTORY OXIDE SELECTED FROM THE GROUP CONSISTING OF ZIRCONIA AND THORIA, STABILIZED, CONTAINING FROM 3% TO 6% OF ALKALINE OXIDE SELECTED FROM THE GROUP CONSISTING OF LIME AND MAGNESIA, SAID TUBULAR ELEMENT BEING OF SUBSTANTIALLY UNIFORM CROSS SECTION THROUGHOUT ITS EFFECTIVE LENGTH AND HAVING A WALL THICKNESS NOT GREATER THAN THAT AT WHICH, THROUGHTOUT THE DESIRED OPERATING TEMPERATURES AND IN RESPONSE TO CURRENT FLOW THEREIN, HEAT IS GENERATED INTERNALLY OF THE WALL MASS OF SAID STABILIZED REFRACTORY OXIDE AT A RATE GREATER THAN THE RATE AT WHICH SAID TUBULAR ELEMENT LOSES 